DRAG DROP
You have an Exchange Server 2013 organization that contains two servers named EX1 and EX2.
EX1 and EX2 are the members of a database availability group (DAG) named DAG1. DAG1 contains a
database named DB1. DB1 is active on EX1.
You deploy a new Exchange Server 2013 server named Ex3.
You add EX3 as a member of DAG1.
You need to add a copy of DB1 to EX3 by using an offline copy of the database.
Which three actions should you perform in sequence? (To answer, move the appropriate three
actions from the list of actions to the answer area and arrange them in the correct order.)
Answer: 
It looks like the first 2 steps are correct but the last one is update rather than add…
https://technet.microsoft.com/en-us/library/dd351100(v=exchg.150).aspx
The first step is correct. When you run the Add-MailboxDatabaseCopy cmdlet with the -SeedingPostponed switch it creates the copy object and a handful of files in the folder structure. You would then manually copy the database files and overwrite whatever is there. You’d mount the database after that (content index will be Crawling) and use Update-MailboxDatabaseCopy -catalogonly to accelerate the seeding of the index. I would go with:
Dismount DB1 on EX2
Run the Add-MailboxDatabaseCopy cmdlet
Manually copy the database files to EX3
The exact link provided by Joe gives the answer, and it is
Dismount
Copy files
Add-MailboxDatabasecopy (indeed using SeedingPostoned switch as said by testdude)